Pressurized water nuclear reactors generally comprise a vessel enclosing the core of the reactor which is immersed in pressurized water for cooling the reactor.
The generally cylindrical vessel of the reactor comprises a head in the form of a spherical cap which is added onto its upper part, so that the head presents a convex outer upper surface. The head is pierced with apertures at the level of each of which is secured by welding, in vertical position, a tubular penetration piece constituting an adaptor ensuring the passage of an extension of a cluster for adjusting the reactivity of the core or the passage of a measurement means to the interior of the core such as a column of thermocouples.
Each of the tubular penetration pieces is engaged in an aperture passing through the head of the vessel and is welded onto the concave lower surface of the head by an annular weld.
The adaptors intended to receive an extension of a cluster for adjusting the reactivity of the core have an upper part projecting above the head of the vessel onto which is secured a control mechanism serving to move the extension securely attached to the control cluster termed control rod.
Inspections carried out on the nuclear reactors in service have shown that some of the adaptors passing through the head of the vessel have cracks, after a certain head operating time, in particular in the weld zone of the adaptors, in the vicinity of the concave lower surface of the head. These cracks are liable to develop until they pass right through the wall of the adaptor. In this case, a limited but troublesome leakage of primary fluid constituted by pressurized water may occur, to the outside of the head, through the crack and the space included between the adaptor and the passage aperture of the head. The pressurized water constituting the leakage liquid which is at very high temperature and which finds itself depressurized evaporates, with the result that an escape of steam occurs above the head. The steam carries away certain radioactive fission products which are liable to be found again in the atmosphere above the vessel of the nuclear reactor.
It is necessary to detect very quickly the presence of an adaptor for passage through the head of the vessel having a crack right through it leading to a leakage of primary fluid, in order to effect a repair after shutting down the nuclear reactor in order to prevent the release of gaseous radioactive substances into the enclosure for confinement of the nuclear reactor.
In order to prevent a major transfer of heat between the head of the vessel which is at high temperature and the control mechanisms disposed at the upper part of the adaptors, and in order to protect the latter from too high a temperature, an envelope termed casing in heat-insulating material is placed above and caps the head of the vessel, inside a cylindrical skirt surrounding the adaptors.
So that the casing can be engaged between the adaptors, it is constituted by detachable modular members which, assembled, form a cylindrical envelope with a circular horizontal plane at the upper part. The casing thus constituted is pierced with large apertures so that in position on the head of the vessel it does not ensure leaktightness around the adaptors. The casing does not therefore produce any confinement serving to retain or possibly to detect leakage gases above the head of the vessel.
When the casing is constructed from members such as panels inserted between the adaptors and assembled to each other, these panels are not connected to one another in a leaktight manner, with the result that the casing does not provide any improvement where safety and the monitoring of leaks is concerned.
In a more general way, when a receptacle or pipeline receiving a fluid at high temperature and possibly at high pressure is surrounded by an enclosure or envelope in heat-insulating material, this envelope or enclosure does not serve to confine or to retain a gas which may escape from the receptacle or from the duct in the case of leakage. Neither does the envelope or enclosure serve to detect leakage in the vicinity of the outer surface of the receptacle or of the pipeline in the thermally insulated zone.